Did you ever get a solution to your problem? Because I just hit the exact same problem and I haven't found a way around it yet. I'd be very curious what you ended up doing.
To restate: Google Sites will let you set up a "URL mapping" for any name within the domain you're hosting, to point to a specific place in Google Sites. So if you're hosting "foo.com" in Google Apps, you could make "http://www.foo.com/" point to a specific "site" in Google Sites. (assuming you have the proper DNS records set up). But -- and this is a limitation of Google Sites, not of Dreamhost -- you cannot make "http://foo.com/" point anywhere in Google Sites.
I tried, in Dreamhost, putting redirect-style hosting on "foo.com" and setting it to redirect to "http://www.foo.com/", but that tried to insert a DNS record for www.foo.com, as well, which clashes with the CNAME to ghs.google.com. We need a way to redirect only foo.com, and leave www.foo.com as a CNAME to google.